1. Technical Field of the Invention
This invention relates to a communication system for transmitting and receiving communication data together with signature data attached thereto for verifying the communication data.
2. Related Art
In recent years, there has been considered introduction of a system that is operative to exchange vehicle information about one's own vehicle, such as a vehicle location, a vehicle velocity, a travelling direction, and ON/OFF operation of a brake, with peripheral vehicles around the own vehicle through inter-vehicle communications to notice the driver of a risk of collision with one or more of the peripheral vehicles, thereby avoiding an upon-meeting collision at a blind intersection before the collision occurs. Japanese Patent Application Publication No. 2009-081524 discloses such a system.
In the above system, one of the main objects is how to assure validity of communication data. There is known as a solution an electronic signature scheme based on the public key cryptosystem.
In the electronic signature scheme, the communication data is to be transmitted with attached data for the electronic signature (hereinafter, referred to as signature data), thereby enabling the public key to be verified and checking whether or not the communication data has been tampered with by means of a message digest generated by a hash function.
In the electronic signature scheme based on the public key cryptosystem, however, a larger public key is required to assure higher reliability, which leads to larger signature data.
There exists a following disadvantage in the above scheme. In cases where a relatively small amount of communication data is allowed to be transmitted/received in each communication and is comparable in data amount to the signature data, an overhead for the signature data is increased, which leads to reduction in communication efficiency (see FIG. 8).
In particular, in the above inter-vehicle communication system, regular exchange of data is required. In addition, data to be exchanged (i.e., the vehicle information) in each communication is assumed to be about 100 bytes in size, while the signature data is 200 bytes or more in size. That is, when the signature data is attached to the data to be exchanged, a total amount of data required for each communication will be increased two or three fold.
An amount of data that can be exchanged in each communication (i.e., size of one communication frame) is determined by a baud rate and an allowable number of concurrent communication partners and other conditions.
A radius of an inter-vehicle communication area is here assumed to be 200 m, whose center is located at a traffic intersection of 4 roads each having 3 lanes on one side. Assuming that a plurality of vehicles are running spaced apart from each other with an average inter-vehicle distance of 10 m, there exist 20 vehicles per lane within a 200 m long segment of each road. Therefore, 3 lanes on one side×2 lanes on the other side×4 directions×20 vehicles per lane gives 480 vehicles within the area of 200 m radius, of course, which may depend on a traffic condition.
Assuming that each road is a high-speed way where the vehicle speed is 30 m/sec, and taking into account potential data missing due to communication errors or the like, a data transmission cycle is preferably limited to 100 ms or less such that the data missing effect on the communication control is made allowable. In addition, it may be assumed that the baud rate is around 10 Mbps, but which is depends on an allocated bandwidth of radio waves.
Under such a condition, an amount of data that can be transmitted in each communication can be estimated from the following equation.10×106 [bps]×0.1 [s]/480 [vehicles]=2083 [bits]
That is, even in the absence of transmission loss, the data amount can take nothing more than about 260 bytes. In addition, an increasing packet density leads to significant reduction in communication efficiency due to frequent occurrence of packet collisions. Therefore, it is said that the data amount is actually limited to under about 30% of 260 bytes.
That is, a disadvantage of the above inter-vehicle communication system is that since the signature data is about 200 bytes in size, it is probably impossible to transmit even 100 bytes of data that are originally intended to be transmitted.
In consideration of the foregoing, exemplary embodiments of the present invention are directed to providing a communication system that is capable of preventing significant reduction in throughput of communication data due to the additional signature data.